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Abstract
This study deals with the characterization of the basement rocks of Gabal Um Anab area (Lat. 26? 48’ and 27? 00’ N and Long. 33? 25’ and 33? 40’ E). The study is based on field mapping (1:100,000), petrography, geochemistry and radioactivity of the encountered rocks.
Field studies indicated that the area of Gabal Um Anab is intensively covered by granitoid rocks. The basement rocks can be arranged beginning with the oldest as follows: metavolcanics and related rocks, old granitoids, younger gabbros and younger granites as well as post-granite dykes.
The rocks related to the metavolcanics comprise volcanogenic metasediments and metapyroclastics. The volcanogenic metsediments comprise amphibole-bearing schists and quartzofeldspathic schists, whereas the metapyroclastics are banded and include welded metatuffs and crystal metatuffs. The volcanogenic metasediments were subjected to regional metamorphism of the greenschist-low amphibolite facies as indicated from the mineral assemblage; chlorite, actinolite, hornblende and plagioclases. This mineral assemblage suggests the formation of these rocks at temperature between 400 and 450 C and pressures between 4.0 and 5.0 kb.
The metavolcnics are classified into metabasalt, metabasaltic andesite and meta-andesite. The metabasalt and metadolerite were originated from tholeiitic magma developed in a volcanic arc setting. The metabasaltic andesite and meta-andesite were derived from calc-alkaline magma developed in a volcanic arc setting.